I just finished installing another Keel supercharger kit on my car. I had one for 5+ years a while back. Almost no issues with it. Certainly not harsh upshifts from my AT. I have a NOS 89 S4 AT installed less than 9K miles ago when the original tranny failed beyond my repair skills. The new tranny has been wonderful until now.

So, I finished the SC install, did a little Sharktuning, and the car is running great. BUT i noticed partial throttle upshifts are harsh with a decided clunk. Full throttle is firm but I think OK. So, I thought I must have pulled the tran's vacuum line off where it joins the hard line under the firewall. Nope. It holds vacuum. In fact the entire 5-way holds vacuum perfectly, which is amazing since the dampers and regulator are original. So, then I thought I must have the vac source on the wrong port of the 85/6 throttle body used in this install. Maybe ported vacuum by mistake. Nope. In fact it doesn't seem to matter which port I use, the upshifts are harsh on partial/no throttle. Also, surprisingly to me the drop into 1st actually is smooth. It usually isn't with these trannys.

What's going on? What did I miss? The Bowden is set OK, although that shouldn't matter. I did no fiddling back at the tranny, so I don't understand why this issue suddenly appeared. I could try to adjust the modulator, but why should I have to suddenly do this? One remote possibility is a blocked vac line to the tranny, but really? The fact that the vac port used doesn't seem to matter would point to this. Maybe I should pull the trans vac line off the 5-way and go for another ride.

Puzzled.

Help appreciated.

 

Bill, doesn`t that trans have an adjustable modulator on the side. Used to adjust shift points and harshness? I remember having to make adjustments after installing a supercharger. Kevin

 

Hi Kevin: Yes, it does. But I sure don't recall having to do any adjustments before. I looked at the vac at partial throttle and it is very good..maybe 25 in. while about 18 at idle. The supercharger is downstream from the throttle so boost should if anything raise vac as it sucks air through the partially open throttle. Do I have this wrong? I may have to give up and do an adjustment. I wonder if the SC bypass is not installed right, although that is pretty simple, and I seem to have good vac on partial throttle at the throttle regardless.

 

Just a WAG but with the vacuum being that high at part throttle is the trans shifting as if the throttle is barely off idle when in fact the system is under significant load?

 

Can you put a vacuum gauge on the line to the transmission and see what it's doing while you drive around?

 

Sounds like the modulator isn't getting vacuum. My 89 acted the same way with the vacuum disconnected after I replaced the broken modulator with little change (pressures were set correctly)

Check to see if the modulator itself holds vacuum, it may have failed but I doubt it with how new the entire trans is. The lines on the firewall can melt shut if they get too hot as depicted in my recent thread https://rennlist.com/forums/928-foru...cuum-line.html

I thought it was fine because it held vacuum as well, but it turned out literally everything had failed in my case.

 

Hi Blake. Yes, it's acting like the modulator is not getting vacuum. The line going back to the tranny holds vacuum, so that would argue the modulator is holding vacuum unless the line itself is blocked, as you found, and which I recognize is a possibility. It just seems odd for it to pop up coincidentally with the SC install. The line going down to the metal section above the bell housing is silicone tubing and I can see all of it. I guess I'll have to pull off the air inlet housing from the rear of the SC and get down to the metal part of the line. Thanks for your input.

 

I suppose I could. Let me get 10 or 15 ft of vac line and a T. That would be a direct way to see what's happening at the tranny. Plus I could check the modulator directly while I am down there. As nothing at the engine end seems to point to a problem, I will probably take your advice.

 

Bill,

Those vac figures look suspect to me- surely you should pull max vacuum at idle? With the positive displacement supercharger are you overwhelming the venturi system by any chance? More to the point how/where is it connected now?

 

Fred, you lost me. This is a roots SC. I can't recall any similar problem when I had the same installation some 5 years ago. What veturi are we talking about? The throttle and MAF are before the SC. The vacuum source for the 5-way is hooked to the 85/86 throttle housing. It has 2 ports. Maybe you can educate my how they differ. I note on one the vac falls with throttle opening, while on the other it increases. It actually doesn't seem to matter in terms of shift quality as to which I use.

 

Bill,

The venturi provides a permanent bypass around the throttle body and generates a deep vacuum all the time so that the brake booster is always immediately available. The venturi itself is that little Y-piece that sits on the driver side of the stock S4 intake plumbed to the brake booster.

 

OK, I know what you are talking about. That's gone in this SC installation. The brake booster gets fed from a port (venturi) in the intake adapter on the back of the SC. The 5-way vacuum connector that serves the transmission, the fuel dampers and regulator and the air pump diverter is fed by one of the ports on the 85/86 throttle further forward in the air intake. So, I don't see the connection between the brake booster, it's venturi vacuum source and the transmission vacuum problem I seem to be having.

 

Biil,

The vacuum should be deeper [i.e. bigger number] at idle than it is at part throttle. For purposes of clarity, the amount of vacuum pulled is the pressure difference between atmospheric pressure and the absolute pressure within the manifold or inlet tract depending on where the measurement reference takes place. If the vacuum is not as deep as it should be during the change process then the shift will be harsher but then I am pretty sure you will fully understand this concept. My query not knowing your configuration was whether the venturi was being fed pressurised air from the supercharger thus defeating the effect of the throttle plate being closed given the Rootes blower will be on boost long before a centrifugal supercharger spools up [typically 3k rpm]..

I assume the air flow path goes something like air filter, MAF, compressor, throttle body and then the inlet tract/runners.

 

Hi Fred. The vacuum figures I reported were on one of the two ports on the throttle housing. I think that one is ported vacuum, which rises up to a certain RPM range. The other port does as you say, falling off, once the throttle is open. Regardless, the shifting doesn't seem to be different with either of these ports.

 

Ockham’s Razor: blocked, collapsed vac line.
Pull the vac line off the transaxle and at the 5-way. Apply vacuum hand pump. Line should hold no vacuum if not blocked.

If not blocked, then drive around with vac gauge...